This invention relates to apparatus and method for removing tramp metal in the form of ferrous metal particles from the plastic material being fed from a hopper and through a feeding passageway to a screw extruder positioned therebeneath.
Many problems are brought about by tramp metal which consists primarily of ferrous metal particles fed along with plastic material as to the screw or ram screw of an extruder. The housing or barrel and the screw itself may become damaged by the grinding action induced by the tramp metal. This grinding action is the reason for severe wear upon the screw assembly including the housing with excessive wearing and breaking of the components of the screw extruder. Moreover, the quality of the extruded product may be impaired by the presence of tramp metal which passes through the process. The normal cycles of operation of a screw extruder are often interrupted in an attempt to remove tramp metal as well as to carry out routine cleaning by the insertion of a horizontal slide plate across the feeding passageway to temporarily interrupt the flow of plastic material from the hopper into the screw extruder. In customary operation the housing or barrel of the screw extruder is surrounded by spaced band heaters and the like for melting the plastic material to be pushed as by the combination of screw and ram or screw alone through a die or to a mold which configures the extrusion into a desired shape or cross section.
Efforts have been made to remove tramp metal and any other magnetic metal material flowing from a hopper through a feeding passageway such as is illustrated in U.S. Pat. No. 4,333,825. The difficulty of mounting a magnet of sufficient size to effectively remove the magnetic particles is accentuated by the difficulty of removing a non-metallic cover while the over sized magnet is retained within the passageway. U.S. Pat. No. 4,867,869 illustrates a grate magnet which extends across a feeding passageway utilizing bar magnets and a retaining tube assembly which are withdrawn from the feeding passageway sequentially in a horizontal plane but are not completely removed therefrom. This structure causes difficulty in removing the tramp metal from the grate magnet apparatus which is retained partially within the passageway or housing through which the flowing material containing tramp metal passes. The prior art is further illustrated by U.S. Pat. No. 5,043,063 and Fed. Rep. of Germany Pat. No. 3,307,319, 9/1984.